Decrease in serum levels of 1,25-dihydroxycholecalciferol in rats and chickens fed a vitamin D-deficient diet.

Twenty-one-day-old rats placed on a vitamin D-deficient diet showed no decrease in serum, 1,25-dihydroxy vitamin D levels after 13 days on this diet. Between 13 and 20 days on this D-deficient diet there was a 50% decrease in serum 1,25-dihydroxy vitamin D. After 34 days, the level of 1,25-dihydroxy vitamin D in serum had dropped to near zero. With a vitamin D-deficient diet lacking calcium, there was an apparent stimulation of 1-hydroxylase, resulting in higher 1,25-dihydroxy vitamin D serum levels after 6--20 days on the diet. After 27 days there were no differences in 1,25-dihydroxy-vitamin D levels in animals fed a calcium-replete or calcium-deficient rachitogenic diet. When 1-day-old chickens were maintained on a rachitogenic diet for 1 week, 1,25-dihydroxy vitamin D levels were higher in animals fed the calcium-deficient diet compared with the calcium-replete diet. After 2 weeks on either rachitogenic diet, 1,25-dihydroxy vitamin D levels decreased to near zero. Measurement of 1,25-dihydroxy vitamin D levels has provided a biochemical indicator of vitamin D deficiency in chicks and rats which will complement other established biological criteria for vitamin D deficiency.